This passage is a mention of the germ theory of disease. Cochran suggests that there is a lost manuscript actually discussing it, intended for people who don't already know the theory.
I get that, but germ theory during the middle ages wasn't exactly widespread, was it? If I'm not mistaken, the dominant belief was that diseases were caused by foul-smelling odors, not germs.
Imagine reading Rerum rusticarum libri III as a medieval scholar, or maybe event the relevant source. Could we expect a medieval scholar to jump and shout: "Yes! Yes! Of course! Disease is caused by small invisible creatures, not water that smells bad."
That's what Jack is trying to say, I think. With knowledge (and even proof) of germ theory, a find like th...
Related: Son of Low Hanging Fruit, Low Hanging Poop
A post by Gregory Cochran's and Henry Harpending's blog West Hunter.